The present invention relates to a method for producing electrochemical gas sensors and is directed more particularly to a method for producing electrochemical gas sensors that include a plurality of closely spaced electrodes.
In measuring the concentrations of electrochemically reducible gases such as oxygen, it is a common practice to utilize membrane-type gas sensors. Sensors of the latter type include a gas-permeable membrane that is located at one end of a chamber which is filled with a suitable electrolyte solution, such as an aqueous solution of potassium chloride. Immersed in the electrolyte solution is an anode electrode and a cathode electrode. Of these, the cathode electrode is usually located adjacent to the membrane to maximize its exposure to the gas to be measured. These electrodes are connected to a remote instrument console which applies a polarizing voltage to the electrodes and provides a user-readable indication of the current that flows therebetween.
When oxygen is being measured, for example, oxygen molecules diffuse through the membrane and are reduced to hydroxyl ions in the layer of electrolyte that lies between the membrane and the cathode electrode. As this occurs, a corresponding oxidation reaction occurs at the anode electrode. As the oxygen in this layer of electrolyte is reduced, an oxygen concentration gradient is established between it and the main body of the electrolyte. This concentration gradient, in turn, causes any oxygen that is dissolved in the main body of electrolyte to diffuse toward the cathode electrode. As this dissolved oxygen arrives at the cathode it too is reduced, thereby giving rise to an error in the measured oxygen concentration. The part of the sensor current which is attributable to these diffusing gas molecules represents one component of the residual current of the sensor.
In order to reduce the effect of such residual currents, some gas sensors are provided with guard electrodes which serve to reduce diffusing gas molecules before they arrive at the cathode. One guard electrode of this type is shown in U.S. Pat. No. 3,454,485, issued in the name of Hauk et al., on July 8, 1969.
Guard electrodes have also been used in transcutaneous oxygen sensors. An example of a sensor of the latter type is shown in U.S. Pat. No. 4,324,257, issued in the name of Albarda et al., on Apr. 13, 1982.
While gas sensors having guard electrodes operate satisfactorily, they are expensive to produce. The reason is that guard electrodes and the mounting structures that are used to support them are bulky. As a result, gas sensors that are to include guard electrodes may have to be totally redesigned to accommodate them. This, in turn, may involve the cost of making new injection molds and/or extensive machining.